I have also had success with this. When the 7 came out, I purchased it and decided a few weeks after my return window was up that I would rather have the 7 plus. I reached out to Apple, explained the situation, and was given a full price credit for my 7 on trade in for the 7 plus - meaning I effectively just had to pay the difference between the two phones to upgrade.I have returned many iPhone's that are outside the 14 day period. Call their customer service or start a chat from Apple's website. Ask them for a onetime return and let them know you are purchasing another iPhone and I am sure they will send you a return label. You have nothing to loose.
Not sure if they still make exceptions like this, but worth a try.